Difficulty has been experienced in furnace-dryer systems in handling waste wood products, and particularly those containing bark, and particularly stringy, wet bark having a significant amount of uncombustible particles, such as dirt. Most of this type of waste wood has been simply burned to get rid of it. In the following description, the term "waste wood" will mean the residue of wood products from the ordinary lumber mill operation, including sawdust, bits, pieces and scraps of wood and bark.
Attempts have been made to utilize waste wood in a furnace-dryer environment. One type of apparatus provides a heat exchanger between a veneer dryer and the furnace fuel cell. Or a collector has been interposed between the furnace and dryer, but a significant pressure drop results. Or a special and expensive fan has been interposed between the furnace and the dryer to provide sufficient pressure to force the combustion gases toward the dryer. I am also aware of two other waste wood systems, one in which I understand burning is done at atmospheric or sub-atmospheric pressure and fuel is dumped onto the pile. In another, a hot sand bed is employed. The system has little or no turn-downs, fires at maximum rate, venting gases when load reduces or needs to change, and feeds fuel from the top of the furnace fuel cell.